Solid imaging apparatus and method with coating station

ABSTRACT

A solid imaging apparatus and method produces an integral three-dimensional object from a multiplicity of cross sectional portions of the object by selectively exposing successive layers of a liquid photoformable composition to actinic radiation. The apparatus includes a vessel for containing the composition so as to present a free surface, and a movable platform disposed within the vessel below the free surface. Part of the composition is transferred above the free surface by lowering and raising a dispenser at predetermined positions located away from the platform. A doctor blade contacts the composition transferred above the free surface, and then moves over the platform to form a substantially uniform layer of the composition.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 07/884,030 filed May 18,1992, which is a continuation-in-part of application Ser. No.07/804,269, filed Dec. 5, 1991, now abandoned, which is a continuationof application Ser. No. 07/488,095, filed Mar. 1, 1990, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to a solid imaging method and apparatus forfabricating an integral three-dimensional object from a multiplicity ofcross sectional portions of the object. More particularly, the crosssectional portions correspond to solidified portions of contiguouslayers of a photoformable composition. The method and apparatus use adispenser in a coating station, which transfers part of thephotoformable composition over the free surface of the composition sothat a doctor blade may produce a uniform liquid layer.

2. Description of Related Art

Many systems for production of three-dimensional modeling byphotoforming have been proposed. European Patent Application No. 250,121filed by Scitex Corporation Ltd., on Jun. 6, 1987, discloses athree-dimensional modeling apparatus using a solidifiable liquid, andprovide a good summary of documents pertinent to this art. U.S. Pat. No.4,575,330, issued to C. W. Hull on Mar. 11, 1986, describes a system forgenerating three-dimensional objects by creating a cross-sectionalpattern of the object to be formed at a selected surface of a fluidmedium capable of altering its physical state in response to appropriatesynergistic stimulation by impinging radiation, particle bombardment orchemical reaction. Successive adjacent laminae, representingcorresponding successive adjacent cross-sections of the object, areautomatically formed and integrated together to provide a step-wiselaminar buildup of the desired object, whereby a three-dimensionalobject is formed and drawn from a substantially planar surface of thefluid medium during the forming process. U.S. Pat. No. 4,752,498, issuedto E. V. Fudim on Jun. 21, 1988, describes an improved method of formingthree-dimensional objects, which comprises irradiating an uncuredphotopolymer by transmitting an effective amount of photopolymersolidifying radiation through a radiation transmitting material which isin contact with the uncured liquid photopolymer. The transmittingmaterial is a material which leaves the irradiated surface capable offurther crosslinking, so that when a subsequent layer is formed it willadhere thereto. Using this method, multilayer objects can be made.

A publication entitled “Automatic Method for fabricating athree-dimensional plastic model with photohardening polymer” by HideoKodama, Rev. Sci. Instrum. 52(11), 1770-1773. November 1981, describes amethod for automatic fabrication of a three-dimensional plastic model.The solid model is fabricated by exposing liquid photo-forming polymerto ultraviolet rays, and stacking the cross-sectional solidified layers.A publication entitled “Solid Object Generation” by Alan J. Herbert,Journal of Applied Photographic Engineering, 8(4), 185-188, Aug. 1982,describes an apparatus which can produce a replica of a solid orthree-dimensional object much as a photocopier is capable of performingthe same task for a two-dimensional object. The apparatus is capable ofgenerating, in photopolymer, simple three-dimensional objects frominformation stored in computer memory. A good review of the differentmethods is also given by a more recent publication entitled “A Review of3D Solid Object Generation” by A. J. Herbert, Journal of ImagingTechnology 15: 186-190 (1989).

Most of these approaches relate to the formation of solid sectors ofthree-dimensional objects in steps by sequential irradiation of areas orvolumes sought to be solidified. Various masking techniques aredescribed as well as the use of direct laser writing, i.e., exposing aphotoformable composition with a laser beam according to a desiredpattern and building a three-dimensional model, layer by layer. Inaddition to various exposure techniques, several methods of creatingthin liquid layers are described which allow both coating a platforminitially and coating successive layers previously exposed andsolidified.

The aforementioned methods of coating, however, are not capable ofensuring flat uniform layer thickness or of producing such layersquickly, or they do not effectively prevent damage or distortion topreviously formed layers during the successive coating process and theyinvolve coating only liquid formulations of preferably low viscosity.Furthermore, they omit to recognize very important parameters involvedin the coating process such as the effects of having both solid andliquid regions present during the formation of the thin liquid layers,the effects of fluid flow and rheological characteristics of the liquid,the tendency for thin photoformed layers to easily become distorted byfluid flow during coating, and the effects of weak forces such ashydrogen bonds and substantially stronger forces such as mechanicalbonds and vacuum or pressure differential forces on those thin layersand on the part being formed.

The Hull patent, for example, describes a dipping process where aplatform is dipped below the distance of one layer in a vat. thenbrought up to within one layer thickness of the surface of thephotoformable liquid. Hull further suggests that low viscosity liquidsare preferable but, for other practical reasons, the photoformableliquids are generally high viscosity liquids. Motion of the platform andparts, which have cantilevered or beam regions (unsupported in the Zdirection by previous layer sections) within the liquid, createsdeflections in the layers, contributing to a lack of tolerance in thefinished part. In addition, this method is rather slow.

U.S. Pat. No. 2,775,758, issued to O. J. Munz on Dec. 25, 1956, and theScitex application describe methods by which the photoformable liquid isintroduced into a vat by means of a pump or similar apparatus such thatthe new liquid level surface forms in one layer thickness over thepreviously exposed layers. Such methods have the aforementioneddisadvantages of the Hull method except that the deflection of thelayers during coating is reduced.

The patent issued to Fudim describes the use of a transmitting materialto fix the surface of a photopolymer liquid to a desired shape,assumably flat, through which photopolymers of desired thickness aresolidified. The transmitting material is usually rigid and either coatedor inherently nonadherent to the solidified photopolymer. The methodsdescribed by Fudim do not address the problems inherent in separatingsuch a transmitting material from a photopolymer formed in intimatecontact with the surface of the transmitting material. Whereas theeffects of chemical bonding may be reduced significantly by suitablecoatings or inherently suitable films, the mechanical bonds along withhydrogen bonds, vacuum forces, and the like are still present and insome cases substantial enough to cause damage or distortion to thephotopolymer during removal from the transmitting material surface.

Methods utilizing doctor blades and/or material supply mechanisms havebeen proposed in such publications as Japanese Patent ApplicationPublication numbers 61-114817, 61-114818, and 61-116322. However, thesemethods require an exact amount of material or photoformable compositionto be added in the vessel every time a layer has to be formed. Also,they require the doctor blade or smoothening blade to have a lengthequal to the width of the vessel in order to properly operate. Becauseof this, the systems described in these patents have restrictionsnecessarily confining the photosensitive material between the doctorblade and part of the vessel at all times. Thus, it becomes verydifficult to form a uniform layer in one continuous pass of the doctorblade without ending up with an excess or shortage of material at theend of the pass. In other words, the doctored layer may be eitherlacking a part of it at the end of one doctoring operation or it mayhave an excess of material, which will be very difficult to redistributein order to achieve the proper thickness and uniformity, due to theconfined nature of the arrangement. Also, the doctor blade has atendency to create wave motion in the material surrounding thepreviously exposed layer causing a disturbing effect, particularly onparts of the previously exposed layer which are partially unsupported.

Thus, it is one of the objects of the present invention to provide anapparatus and a method for fabricating an integral three-dimensionalobject from a multiplicity of cross sectional portions of the object,the cross sectional portions corresponding to solidified portions ofcontiguous layers of a photoformable liquid composition, in a fast anduniform manner. Another object of the present invention is to provide agentle way of raising part of the photoformable composition above thesurface of said composition and in front of the doctor blade. Use of apump to recirculate a liquid of the nature used in solid imaging orstereolithography does not present a viable solution because theviscosity and mainly sensitivity of such compositions cause blockage ofthe paths and seizure of the pumping operation at an unacceptably highrate. Premature polymerization within the higher-shear components of thepump seem to be the most probable cause of this problem.

SUMMARY OF THE INVENTION

The present invention comprises a solid imaging apparatus and method forfabricating an integral three-dimensional object by selectively exposingsuccessive layers of a liquid photoformable composition to actinicradiation. The apparatus includes a vessel for containing thecomposition so as to present a free surface, and a movable platformdisposed within the vessel below the free surface. Part of thecomposition is transferred above the free surface by lowering andraising a dispenser at predetermined positions located away from theplatform. A doctor blade contacts the composition transferred above thefree surface, and then moves over the platform to form a substantiallyuniform layer of the composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view showing a preferred embodiment of thisinvention.

FIG. 2 is an elevation view showing a part of the coating stationincluding a dispenser employed in the present invention.

FIG. 3 is an elevation view showing a dispenser for dispensing liquidcomposition.

FIGS. 4 a and 4 b are perspective views illustrating the embodiment ofFIG. 3.

FIG. 5 is an elevation view showing another embodiment of thisinvention.

FIG. 6 is a perspective view showing the embodiment of FIG. 5.

FIG. 7 is an elevation view showing another dispenser where a pivotingmechanism is utilized for dispensing the liquid.

FIGS. 8 a through 8 e are elevation views showing another embodiment ofthis invention.

FIGS. 9 a and 9 b are perspective views showing another embodiment ofthe invention.

FIG. 10 is an elevation view showing a different embodiment of thepresent invention where the dispenser is in the form of a single plate.

FIG. 11 is an elevation view of another embodiment wherein the dispenseris in the form of parallel blades.

FIG. 12 is an elevation view showing the dispenser as a plate having aplurality of bristles attached to its sides so as to have a brush-likeconfiguration.

FIG. 13 a is an elevation view showing still another embodiment of thepresent invention where the dispenser is in the form of a plate having aplurality of pockets on either side.

FIG. 13 b is a perspective view illustrating the embodiment of FIG. 13a.

FIG. 14 a is an elevation view illustrating the use of wipers to controlthe dispensing rate of photoformable composition from a plate-likedispenser.

FIG. 14 b is a perspective view illustrating the embodiment of FIG. 14a.

FIGS. 15 a and 15 b are elevation views illustrating another embodimentof the present invention wherein the doctor blade itself serves as thedispenser.

FIGS. 16 a, 16 b, 17 a, 17 b and 18 are elevation views illustratingother embodiments of this invention where the dispenser has beenincorporated onto the front part of the doctor blade.

FIG. 19 is an elevation view illustrating yet a different embodiment ofthe present invention where the dispenser and the doctor blade have beencombined into a single unit.

FIG. 20 is an elevation view showing another embodiment where thedispenser is always disposed in the liquid and pulsates to form a wavein front of the doctor blade.

FIG. 21 is an elevation view showing an additional preferred embodimentwhere two doctor blades are utilized, with the dispenser located betweenthe two doctor blades.

FIG. 22 is an elevation view showing another embodiment where two doctorblades are utilized with the dispenser located between the two blades.

FIG. 23 is a schematic elevation view showing an embodiment of thepresent invention for lowering and raising the dispenser shown in FIG. 2at predetermined positions and time periods.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a solid imagine method andapparatus for fabricating an integral three-dimensional object from amultiplicity of cross sectional portions of the object. Moreparticularly, the cross sectional portions correspond to solidifiedportions of contiguous layers of a photoformable composition. The methodand apparatus use a dispenser in a coating station, which transfers partof the photoformable composition over the free surface of thecomposition so that a doctor blade may produce a uniform liquid layer.

FIG. 1 shows an imaging station or means 70 including a radiation source10, a modulator 14, a computer 34 and a deflection means 16, preferablyin the form of a scanner. There is also provided a coating station 71.Radiation source 10 is preferably a laser, producing a radiation beam12. In order to produce solid objects at high speed, the imaging station70 preferably utilizes relatively high power radiation sources 10, suchas high power lasers, which may have major bands in the visible,infrared, or ultraviolet regions. For present photospeeds ofphotoformable compositions, high power is considered to be a powergreater than 20 mW, and preferably over 100 mW as measured from theintensity of the radiation beam 12. However, as faster compositionsbecome available, the values of 20 mW and 100 mW for the beam intensitywill become lower accordingly, since photospeed of the composition andintensity of the radiation beam have an inverse relation to each otherin order to achieve the same results. The selection of a certain type oflaser should be coordinated with the selection of the photoformablecomposition in a way that the sensitivity of the photoformablecomposition agrees reasonably well with the wavelength of the laser'semission. Other types of radiation means may also be utilized such aselectron beams, x-rays, and the like, as long as their energy type ismatched with the sensitivity of the photoformable composition, a beam isprovided, and the appropriate conditions for their handling are observedaccording to established ways, well known in the art. Although means maybe provided to modify the shape of the beam cross-section to anydesirable shape, the ordinary shape is circular, and the profile of theintensity of the beam is gaussian with a maximum at the center of thecircular shape.

The radiation beam 12 passes through the modulator 14, preferably anacousto-optical modulator. The modulated radiation beam 12′ passes inturn through the deflection means 16 or scanner, which comprises twomirrors 20 and 22, each mirror having an axis (not shown) allowingreflection of the beam to a free surface 46 in X and Y directions, the Xand Y directions being perpendicular to each other and parallel to thefree surface 46. The mirrors 20 and 22 may rotatably move around theircorresponding axes by means of motors 24 and 26, respectively, forcontrollably deflecting the beam in a vector scanning mode, in the X andY directions, towards predetermined positions of a photoformablecomposition 40 contained in a vessel 44 of the coating station 71. Asthe beam is deflected by the deflection means 16, it assumes anacceleration from zero level to a maximum acceleration, and a velocityfrom zero level to a maximum constant velocity. The velocity andintensity of the beam remain proportional to each other, so that theexposure remains substantially constant. The beam 12″ exposespreselected portions of the composition to a substantially constantdepth as described below.

For the purpose of this invention, the radiation beam 12″ may be notonly a focused beam from a laser, but also light from any other lightsource, modified in a number of different ways. For example, it may betransmitted through any type of variable optical density photomask suchas a liquid crystal display, silver halide film, electro-deposited masketc., or reflected off of any variable optical density device, such as areflective liquid crystal cell. Also, the deflection means may be anyother type of scanner, such as a raster scanner, for example.

The coating station 71 comprises a vessel 44 for containing the liquidphotoformable composition 40. A substantially flat platform 41 isdisposed within the vessel 44 and adapted to be positioned under thefree surface 46 of the composition 40. The platform 44 has sides, suchas a left L and a right R side. A placement means 42 is provided forcontrollably varying the distance between the free surface 46 of thecomposition 40 and the platform 41 through a supporting arm 42′.Although the placement means 42 is shown in FIG. 1 as being fullyimmersed in the composition 40, it should be understood that it may bepreferably positioned outside the vessel, and connected to the platform41 either by passing the supporting arm 42′ through a seal at the bottomof vessel 44, or more preferably by using a bent supporting arm passingaround the vessel and through free surface 46 in order to be connectedto and support the platform 41. An important part of the invention is aspecial dispenser 43 located at one side of the platform 41, in front ofany type of layering means, and preferably between two doctor blades 73and 73′. The dispenser 43 is adaptable to be dipped under the treesurface 46 of the composition 40 and directly transfer part of thecomposition 40 above the free surface 46. The transferred part of thecomposition is used by the doctor blades 73 and 73′ to produce a liquidlayer 48 on top of the platform 41 or on top of previously photoformedlayers. As shown in FIG. 1, communication lines 52, 50, 54, 60, 62, and63 are also provided for the computer 34 to control the radiation source10, the modulator 14, the deflection means 16, the placement means 42,the doctor blades 73 and 73′, and the dispenser 43, respectively.

In operation of the preferred embodiment of this invention, theradiation means 10 provides a radiation beam 12 having an intensity asaforementioned. The radiation beam 12 passes through a modulator 14,where its intensity may be modulated from a zero intensity level to amaximum intensity level having a value less than that of the unmodulatedbeam intensity, due to energy losses. The modulated radiation beam 12′,having somewhat decreased intensity due to losses, passes in turnthrough the deflection means 16 having a two-mirror 20 and 22 assembly,each mirror separately driven by a different motor 24 and 26,respectively. Mirror 20 deflects the beam in a X direction, while mirror22 deflects the beam in a Y direction, the X direction beingperpendicular to the Y direction. Electrical feedback regarding therelative movements of the mirrors 20 and 22 is provided by thedeflection means 16 to the computer 34 through line 54. This feedback,being correlatable to the velocity and average residence time of thebeam 12″ on the predetermined portions of the thin layer 48, isprocessed by the computer 34, and it is fed to the modulation means 14as a control command through line 50 in order to modulate the intensityof the radiation beam 12, so that the product of the intensity of thebeam 12″ and the average residence time at each position of thepredetermined portions of layer 48 remains substantially constant. Thus,the exposure level, being by definition the product of these twoparameters, remains substantially constant. By maintaining the exposurelevel constant over the predetermined portions of each contiguous thinlayer, the thickness of the layers is also kept substantially constant.This correction or compensation is very important, especially atunsupported portions of the thin layers, where swollen edges will appearas a result of overexposure due to the low initial velocity at the edgesin vector scanning. The higher the intensity of the beam 12″, or thehigher the photosensitivity of the photoformable composition, the moresevere this problem becomes in the absence of means to maintain theexposure level constant. Such exposure control is also necessary inraster scanning or in systems incorporating overscanned vector schemes,the difference being that the edges of the image may be underexposed dueto lack of exposure contribution from adjacent non-exposed regions. Inthese cases, modulation means are utilized to ensure that edge regionsof the image receive substantially the same exposure as non-edgeregions. In any event, the radiation beam 12″ is controllably directedtowards the photoformable composition 40.

The platform 41, which has a substantially flat upper surface 41′, isinitially placed within the vessel 44 in a way that the flat uppersurface 41′ is contained within the free surface 46 of the composition40. In sequence, the platform 41 is lowered in the composition 40 by thethickness of the layer 48. The dispenser 43, which is preferably kept atleast partially dipped under the free surface 46 of the photoformablecomposition 40 when not in motion, is raised and starts dispensingliquid composition 40 between the doctor blades 73 and 73′. The doctorblade 73 then produces a uniform liquid layer 48 on top of thesubstantially flat surface 41′ of platform 41. In FIG. 1, the dispenser43 is shown to be partially dipped in the composition 40 adjacent theright side R of the platform 41. When the doctor blades 73 and 73′ andthe dispenser 43 reach the left side L of the platform 41 they stop, andthe dispenser 43 is preferably at least partially dipped in composition40 under the free surface 46. Preferably, the dispenser 43 remainscompletely dipped at this stage. A short time may be allowed, ifnecessary, for the free surface 46 to reach equilibrium and assume thedesired uniformity. At least a portion of the liquid layer 48 is thenexposed imagewise by actinic radiation, which preferably is in the formof the laser beam 12″.

After this fast imaging step, the platform 41 is lowered again by thethickness of the layer 48. The dispenser 43, which was now keptpartially dipped under the free surface 46 of the photoformablecomposition 40 at the left side L of the platform, is raised and startsdispensing liquid composition 40 between the doctor blades 73 and 73′.The doctor blade 73′ then produces a uniform liquid layer 48 on top ofthe platform 41 and previously photoformed layer as the assembly ofblades 73 and 73′ and dispenser 43 now moves towards the right side R ofthe platform 41. When the assembly of the doctor blades 73 and 73′ andthe dispenser 43 reaches the right side R of the platform 41 they stopagain, and the dispenser 43 is dipped in the composition 40 under thefree surface 46. A short time may be allowed again, if necessary, forthe free surface 46 to reach equilibrium and assume the desireduniformity. At least a portion of the liquid layer 48, now being on topof the previously imagewise exposed layer, is exposed imagewise to thelaser beam 12 a. The above steps are repeated until all contiguouslayers have been produced and the three dimensional object has beencompleted. All the above steps are coordinated by the computer 34 in aconventional manner.

In the present invention, the equilibrium level of the free surface 46always remains substantially constant, regardless of the distance movedby the platform, because the amount of photoformable composition 40within the vessel 44 remains the same since no additional composition 40is added. The composition needed for successive layers 48 is transferredabove the free surface 46 by lowering and raising the dispenser 43 atpredetermined positions alongside the platform 41. Since the dispenser43 dips under the free surface 46 and directly transfers part of thecomposition above the free surface 46, the temporary level of the freesurface 46 will be lowered, relative to the previous equilibrium level,due to transfer of some of the composition and the dispenser 43 abovethe free surface 46. However, after the dispenser 43 is again dippedinto the composition 40 below this temporary free surface 46, the freesurface 46 quickly returns to its equilibrium level. Consequently, theequilibrium level of the free surface 46 will always remainsubstantially the same, thereby ensuring that the distance between thedeflection means 16 and the free surface 46 remains substantiallyconstant. It is critically important that this distance remainsubstantially constant in order that the laser beam 12″ remain focusedprecisely at the surface 46 of the composition so as to achievedimensionally photoformed layers. Even though a typical photoformablecomposition 40 may change in volume upon polymerization by shrinkingapproximately one (1) percent, in practice such a change in volume isnot significant and does not require any fine adjustments in theequilibrium level of the free surface 46 or the adding of additionalcomposition 40, particularly when the mass of the object beingfabricated is less than thirty (30) percent of the mass of thecomposition 40 in the vessel 44. Usually, the mass of the fabricatedpart is between one (1) and five (5) percent of the mass of thecomposition in the vessel 44. Du Pont's SOMOSä solid imaging materialsare sufficiently close to “ideal” such that no fine turning of theequilibrium level of the free surface 46 is necessary during thefabrication process. It is also significant in the present inventionthat the dispenser 43 allows the temporary level of the free surface 46to be lowered while the doctor blade 73 moves across the platform 41, sothat the doctor blade 73 minimizes any type of wave motion in thecomposition surrounding the previously exposed layer, thereby preventingany such wave motion from disturbing the previously exposed layer,particularly those parts of the exposed layer which are partiallyunsupported.

FIG. 2 illustrates the usefulness of the dispenser 43. When the platform41 is lowered by the thickness of the layer 48, the composition 40 doesnot form a complete layer on top of previously solidified layers 11.Thus, unless the three-dimensional object has very limited dimensions,only a small part of the surface of the most recently solidified layeris covered by the liquid layer 48, while a remaining part 48′ of thesurface remains uncoated. Any conventional means may be used to lowerand raise the dispenser 43 at predetermined positions and time periods.Such means include but are not limited to motors combined with rails inthe form of cams, electromagnets and the like. One example isillustrated schematically in FIG. 23. In the embodiment shown in FIG.23, a rail cam 2393 is employed to lower and raise the dispenser 2343under and above the free surface 2346 of the photoformable composition2340, respectively. The dispenser 2343 is also shown with dotted lines,adjacent the right side R′ of the platform 2341, where it has beenlowered due to the shape of the rail cam 2393, the path of which isfollowed by cam follower 2394. It is understood that the dispenser willalso be lowered adjacent the left side L′, and raised in theintermediate position as shown by the complete lines.

In operation, referring back to FIGS. 1 and 2, the dispenser 43 isdipped into the photoformable composition 40 and then it is raised abovethe free surface 46 of the composition 40. As soon as the dispenser 43is raised above the free surface 46 of the composition 40, thecomposition 40 from the dispenser 43 starts being dispensed in front ofthe doctor blade 73, preferably by force of gravity. It is veryimportant that there be an abundance 74 of the composition 40 in frontof the doctor blade 73 so that a complete layer 48 may be formed. Toavoid entrapment of air in the form of air bubbles in the abundance 74of composition 40 in front of the doctor blade 73, it is important thatthe dispenser 43 be raised only slightly above the free surface 46 ofthe photoformable composition 40. It has also been found that thecontinuity of the liquid dispensed as a curtain or extrusion sheet bythe dispenser 43 may suffer interruptions and splitting if the distancebetween the dispenser 43 and the free surface 46 is high. This willdepend, however, on the Theological characteristics of the photoformablecomposition 40, the characteristics of the dispenser 43, and otherfactors. Thus, it is preferable in general that the dispenser 43 beraised by less than 5 mm, more preferable by less than 2 mm, and evenmore preferable by between 0.5 mm and 1 mm above the free surface 46 ofthe composition 40. It should be noted, however, that during dispensing,the gap between the free surface 46 of the composition 40 and thedispenser 43 should be equal to or greater than the gap between the samefree surface 46 and the corresponding doctor blade 73.

After the dispenser 43 has been raised, both the dispenser 43 and thedoctor blade 73 move forward with the dispenser 43 leading and thedoctor blade 73 following. The distance between the previouslysolidified layers 11 and the doctor blade 73, when the doctor blade 73is passing above the solidified layers 11, is maintained constant andcorresponds to about the thickness of the layer 48. After a full pass, ashort time may be allowed for the surface 46 of the composition 40 tostabilize, after which the step of exposing image wise is performed. Thespeed of travel of the assembly of dispenser 43 and doctor blade 73should be lower than a certain limit in order to avoid air entrapment inthe form of bubbles. This limit depends on the rheological and foamingcharacteristics of the photohardenable composition 40. With thephotohardenable compositions employed by the applicants, speeds of lessthan 1 inch per second, and preferably about 0.5 inch per second areadequate to cause only minimal air entrapment.

The dispenser 43 may be shaped like a trough as shown in FIGS. 3, 4 aand 4 b. It has a slot 80 at the bottom so that liquid can freely runthrough. Depending on viscosity of the composition 40, the slot 80 canbe thinner or wider in order to deliver a proper amount 74 ofcomposition 40 in front of the doctor blade 73. It may also have theform of a plurality of openings in proximity to each other. During thedipping operation, it is preferable that the dispenser 43 is not dippedcompletely under the free surface 46 of the composition 40, so that thecomposition 40 enters the dispenser 43 through the slot 80, or ingeneral through the bottom opening in whatever form it might be, and notover the top, in order to avoid air entrapment, especially withcompositions of high viscosity. The dipping and raising rates are alsoimportant for preventing air entrapment, and should be adjusteddepending on the viscosity, surface tension and, in general, the foamingcharacteristics of the composition 40. As shown in FIGS. 5 and 6, avalve 82 may be incorporated in the design of the dispenser 43 so thatit can control the slot 80 according to the desired delivery rate forthe composition 40; In this particular example, illustrated in FIGS. 5and 6, this valve 82 can comprise just a rod which can be moved closeror further away to slot 80 in order to permit more or less material togo through at the desired rate.

In another embodiment of the present invention, illustrated in FIG. 7,the dispenser 743 is connected through a hinge 775 so that when thedispenser 743 is to be dipped, it is in an upright position so as toaccept the liquid photoformable composition 40 within its cavity;however, when it is raised and ready to deliver the liquid composition,it is pivoted and inclined around hinge 775 as shown in FIG. 7.

FIG. 10 illustrates another embodiment of the invention where thedispenser 1043 in front of the doctor blade 1073 is just a single plate.This arrangement is especially useful when the viscosity of thecomposition is high enough so that adequate material is attached toblade 1043. The material is then delivered in front of the doctor blade1073 by force of gravity.

Still another embodiment is shown in FIG. 11, where the dispenser 1143comprises a plurality of parallel plates connected at the top. Thisarrangement of the dispenser 1143 allows air to leave the system whileit is being immersed into the composition and also allows air to entersystem when the composition is being disposed through holes 1183perforated at the top connecting the parallel plates.

Dispenser 1243, in a different embodiment shown in FIG. 12, may alsohave the form of a brush so that it can accommodate more liquid.

Still another form of dispenser 1343 is shown in FIG. 13. In this casethe dispenser 1343 has the form of a plate with a plurality of pockets1384 having, preferably, holes 1385 at the bottom of each pocket 1384.

FIGS. 8 a through 8 e illustrate still another embodiment of thisinvention. In this embodiment there are two plates 885 and 886,initially substantially parallel to each other. As shown in FIG. 8 a,the plates 885 and 886 are initially outside the liquid. Then (FIG. 8 b)the plates 885 and 886 are dipped into 5 the liquid, while they arestill kept substantially parallel to each other. The reason for thisparallel configuration is to avoid turbulence and air entrapment withinthe composition 840 while the dispenser is being dipped. After theplates 885 and 886 have been dipped into the composition 840 (FIG. 8 c),they are pivoted to assume a V shape so that their bottoms meet whilethe top parts of the plates 885 and 886 still remain open. FIG. 8 dshows the plates 885 and 886 in the V position outside the liquidcomposition 840. This is followed by slightly opening the bottom partsof the plates to form a narrow slot and allow the composition to bedispensed through the slot (FIG. 8 e). The configuration of these twoplates 885 and 886 is better shown in FIGS. 9 a and 9 b. Plate 885 hastwo pivoting pins 888 and 888′ at the top. Plate 886 has two side walls887 and 887′ which have in their upper corners holes 889 and 889′ whichare adaptable to receive pivots 888 and 888′. Thus, the two plates 885and 886 may be assembled in the form of a dispenser, by pivoting plate885 around pivots 888 and 888′, which in turn are inserted in holes 889and 889′, respectively. The plate 885 may seal the dispenser by closelycontacting the walls 887 and 887′. In this configuration, if so desired,a slot may be formed at the bottom of the plates, or the plates may bearranged to be substantially parallel to each other, or the whole devicecan be in a closed position and not allow liquid to pass through, whenthe bottoms of the plates are brought into contact. Leakage of smallamounts of liquid through the walls of the plate 886 and the sides ofthe plate 885 is not of consequence in most instances. Gasketingmaterials may be used on the sides and bottom of the plate 885, and/orthe walls and bottom of the plate 886, if desired, for better sealing.The walls 889 and 889′ may be replaced by a flexible or elastic materialfor connecting the sides of the two plated in a substantiallyliquid-proof manner. Any conventional mechanism may be used for openingand closing the blades. Such mechanisms may include but are not limitedto bars, wires, and the like, connected to the plate 885 Forcontrollably changing the position of one plate with respect to theother.

FIGS. 14 a and 14 b illustrate an additional embodiment of thisinvention, where the dispenser 1443 is in the form of a plate. Wipers1492 and 1492′ are provided to controllably dispense picked-upcomposition by wiping such composition downward. Thus, when the plate1443 has been dipped and raised in front of the doctor blade (the doctorblade is not shown in FIG. 14 a for clarity purposes), the wipers 1492and 1492′ start moving downward at a desired rate, preferably controlledby the computer 34. Similar results may be achieved by holding thewipers 1492 and 1492′ at a constant distance from the free surface 46 ofthe composition 40, and raising the dispenser plate 1443 at a desiredrate. Of course, this rate may be variable to better fit the dispensingrequirements of the particular case. FIG. 14 b shows a perspective viewof the dispenser 1443 in plate form in combination with one of thewipers 1492′.

As shown in FIGS. 15 a and 15 b, the doctor blade itself 1573 can beused also as a dispenser. The doctor blade 1573 is dipped into thecomposition 1540 and then it is raised to its initial position over thefree surface of the photoformable composition, as shown in FIG. 15 a.Since the doctor blade is going to form the coating by moving in thedirection of arrow A, it is desirable to have a wiper 1592 which movesdown and wipes off the excess composition gathered on the back side ofthe doctor blade 1573. This is to avoid dragging any material behind thedoctor blade 1573 and destroying the uniformity of the produced layer.FIG. 15 a shows the wiper 1592 taking off the excess composition. FIG.15 b shows the same doctor blade 1573 after it has already starteddoctoring and leveling a layer 1548 of photoformable composition 1540,while the wiper 1592 has been lowered and positioned around the freesurface of the photoformable composition 1540. A different way tominimize accumulation of excessive amounts of composition on the backside of the doctor blade 1573 is to dip the blade as it is moving,preferably at an angle smaller than 90 degrees, which increases withtime and finally assumes a value of substantially 90 degrees before thedoctor blade moves on top of the photoformed layers. This angle is theangle formed between the plane substantially containing the front sideof the doctor blade and the plane substantially containing the freesurface (FIG. 1) of the photoformable composition.

The dispenser 1643 shown in FIGS. 16 a and 16 b may be part of thedoctor blade 1673 and located in the front part of the doctor blade1673. The wiper 1692 performs the same task as described in previousembodiments. FIG. 16 b illustrates the doctor blade 1673 after it hasbeen moved from its initial position, and also shows the wiper 1692 inits lower position.

The front of the doctor blade 1743, as shown in FIGS. 17 a and 17 b, mayhave pockets similar to the ones shown in FIGS. 13 a and 13 b for thedispenser 1743. According to this embodiment, the doctor blade 1743 isdipped into the composition and then raised while the wiper 1792 isstationary near the free surface of the photoformable composition so asto immediately wipe off any excess material on the back part of theblade 1743. It should be understood that the embodiments alreadydescribed or the ones described below can be combined in part or inwhole. For example, in any of these embodiments the wiper may be eitherstationary or movable.

Another embodiment, shown in FIG. 18, illustrates the dispenser 1843being part of and located in front of the doctor blade 1873, while thewiper 1892 is stationary and at the bottom of the doctor blade 1873 whenthe blade 1873 is in its initial position.

In still another embodiment illustrated in FIG. 19, the dispenser 1943and the doctor blade 1973 can be combined into a unit having a commondispensing tip 1995 at the bottom of the combined device. In FIG. 19,this combined device is shown while it is providing a layer, while thewiper 1992 has already wiped the back of the device and is in a loweredposition.

FIG. 20 shows still another embodiment where the dispenser 2043 is inthe form of a plate which is continuously dipped into the composition2040. The dispenser 2043 is provided with a pivot 2075 around which theplate may give an upward pulse. Just before the doctor blade 2073 startsits movement for producing a layer, the dispenser 2043 provides anupward pulse and produces an abundance of material 2074 in the form of awave in front of the doctor blade 2073. Frequently, this amount ofmaterial is adequate for a complete layer to be formed. This embodimentworks at its best when the platform and the three-dimensional object tobe formed are not excessively large.

Two doctor blades 2173 and 2173′ may be used, one on each side of thedispenser 2143, as illustrated in FIG. 21, so that a new layer may beproduced regardless of the direction in which the doctor blade/dispenserassembly moves. When the assembly moves in the direction of the arrowsA, the abundance of material is gathered mainly in front of the blade2173, and a new layer is formed behind the blade 2173. Similarly, whenthe assembly moves in a direction opposite to that of the arrows A, theabundance of material is gathered mainly in front of the blade 2173′,and a new layer is formed behind the blade 2173′. It is important thatwhen the assembly stops on either side of the platform, the dispenser isdipped in the liquid composition 2140 to be refurnished with a newamount of photoformable composition 2140 for the next cycle, in theopposite direction.

The embodiment illustrated in FIG. 22 also involves a double doctorblade arrangement (2273 and 2273′) similar to that shown in FIG. 21,except that the dispenser 2243′, in this case, is always disposed in theliquid photoformable composition 2240. There is one dispenser 2243′ onone side of the platform and one dispenser 2243 (not shown) on the otherside of the platform. Their position is located within the composition2240, and under the “stop” positions of the doctor blades on either sideof the platform. Just before the doctor blades start their travel, therespective dispenser moves quickly upward and forces an abundance ofliquid to gather between the two doctor blades. The rest of theoperation is substantially the same as in the other embodiments.

In the cases where the dispenser and the doctor blade are separateunits, it is often desirable to dip the dispenser in a particularmanner. For example, as shown in FIG. 2, it is often preferable to dipthe dispenser 43 at position P2 while the initial position of the doctorblade 73 is on the other side of the platform at position P1. Accordingto this option, the dispenser 43 is moved over to position P2, it isdipped at this position in order to receive an adequate amount ofcomposition, then it is raised and brought back in front of the doctorblade 73 at position P1. During this movement, the dispenser 43 maycontinue delivering material over the platform area. On other occasionswhere the delivery of material can be controlled, such as in theembodiments shown in FIGS. 5, 6, 7 or 8, it may be desirable for thedispenser to start dispensing composition only when it is in front ofthe doctor blade 73 at position P1 and at the same time that the doctorblade 73 is forming the layer. This version of operation may bedesirable when higher accuracy and speed are needed since the materialreceived by the dispenser 43 at position P2 is going to be moved back bythe doctor blade 73.

In most cases it is desirable for the dispenser 43 to be in the dippedposition while exposing the layer, so that material still held by thedispenser will be in the container and will not change the level of thefree surface of the composition and of the layer. As mentioned before,it is important for the dispensed liquid to be very close to the freesurface of the composition in order to avoid splashing and entrapment ofair in the form of bubbles. Of course, one can work uder vacuum, therebymaking the height at which the dispenser operates immaterial.

Since all these devices are controllable by a computer, one can arrangethe delivery cycle as well as their speed of operation in order toobtain maximum efficiency and uniformity. Depending on the viscosity andother properties of the photoformable liquid composition, somewhatdifferent conditions may be needed to obtain optimal results. Sensorssuch as ultrasonic, infrared, and the like may be used to give feedbackto the computer regarding the build-up in front of the doctor blade andregulate, accordingly, the delivery through the above mechanism.

The photoformable compositions which can be used in the practice of theinstant invention are any compositions which undergo solidificationunder exposure to actinic radiation. Such compositions comprise usuallybut not necessarily a photosensitive material and a photoinitiator. Theword “photo” is used here to denote not only light, but also any othertype of actinic radiation which may transform a deformable composition,preferably a liquid, to a solid by exposure to such radiation. Cationicor anionic polymerizations, as well as condensation and free radicalpolymerizations and combinations thereof are examples of such behavior.Cationic polymerizations are preferable, and free radicalpolymerizations even more preferable. Photoformable compositionscontaining thermally coalescible materials are of even higherpreference.

A liquid thermally coalescible photoformable composition is acomposition which solidifies upon exposure to actinic radiation withoutattaining necessarily its ultimate physical properties, particularlywith respect to their adhesive and cohesive characteristics. However, itdevelops adequate integrity to be handled until such time when furthertreatment is provided. The composition is considered as coalescible whenit comprises particulate matter in dispersed form, which particulatematter undergoes coalescence under a certain set of conditions, such asincreased temperature for example. Coalescence is the transformation ofa dispersed phase to a cohesive continuous solid phase.

Preferably the photoformable composition comprises a thermallycoalescible polymeric cohesive material, a photoformable monomer, and aphotoinitiator. Preferably the photoformable material comprises anethylenically unsaturated monomer. Upon exposure to the actinicradiation, the exposed areas of the photoformable composition mustremain thermally coalescible after removing the unexposed areas. This isimportant to improve both adhesion in the joining surfaces between thelayers and cohesion within the layers for a multilayer integral threedimensional object. Actually, cohesive bonds are formed at the joiningsurfaces by the thermally coalescible material, providing superiorproperties to the structure of the final three dimensional object. It isalso very important to prevent substantial overgrowth of infra posedsurfaces, as it will be discussed below.

In the case of photoformable compositions which are not based oncoalescible materials, post treatment after the exposure step is notrequired. In the case where a coalescible material is an essentialcomponent of the formulation, further heat treatment is needed for theobject to attain its ultimate strength. In such cases, when all thelayers of the three dimensional object have been formed by the methoddescribed above,, the unexposed portions of the composition may beremoved by any conventional means, such as shaking the object, blowinggas towards the object, and the like. Further removal may be achieved byrinsing the object with poor, noncoalescing solvents. Water, alcohols,and in general polar solvents are poor solvents for non-polarcompositions and vice-versa. As long as the solvent under considerationdoes not extract excessive amounts of materials from the exposed area orcause the object being rinsed to swell within the rinsing time, it isconsidered to be a poor, non-coalescing solvent. The object then isthermally coalesced in order to develop high cohesive and adhesivestrength. This step may be performed in an oven, such as a convection,IR or microwave oven. Optimum temperature and time are dependent on theindividual composition. Typically the temperature range is 100°-250° C.and the time range is 5-30 minutes. However, temperature and timesoutside these regions may be used.

A very important group of thermally coalescible materials areplastisols. Plastisols are fluid mixtures, ranging in viscosity frompourable liquids to heavy pastes, obtained by dispersing fine particlesize polymeric resins in nonvolatile liquid thermal plasticizers, i.e.,materials which are compatible with the polymer or resin and increaseits workability and flexibility but have no substantial solvent activityfor the resin or polymer under ordinary conditions of storage (e.g. roomconditions). When the plastisol has been formed into a desired shape,e.g., by molding or coating, it can be heated to coalesce the polymericresin particles and the nonvolatile liquid constituent, thereby forminga homogeneous solid mass. Volatile diluents can be added to plastisoldispersions to modify their viscosity and to achieve desirable handlingcharacteristics in coating or other forming operations.

A dispersion that contains no more than 10% volatile diluent is regardedas a plastisol. Since the plasticizer used in the case of plastisolsacts as a plasticizer to solvate the polymer only at temperatures higherthan storage temperatures, it may also be called a thermal plasticizer.The most widely used plastisols on a polyvinyl chloride homopolymer in aplasticizer.

The following photohardenable composition was made by mixing thoroughlythe following ingredients: 1. Ethoxylated Trimethylol Propane  75.0 gTriacrylate 2. Urethane Acrylate Resin  75.0 g (Sartomer 9610) 3.2,2-dimethoxy-2-phenylacetophenone  6.0 g 4. Clear Chem-o-sol ® 7557225.0 g (made by Whittaker Corp. Providence Chemical Division, EastProvidence, RI)

An automobile distributor cap of excellent quality was made by usingthis photohardenable composition, and the method and apparatus asdescribed hereinabove. The double doctor blade arrangement with thedispenser as shown in FIG. 21 was used. When the dispenser was at reston either side of the platform, it was maintained completely immersedunder the free surface of the photohardenable composition. Duringdispensing at the raised position, the dispenser was 1 mm above the freesurface of the composition. The slot width was approximately 0.190 inchand the travelling speed 0.5 inch per second. Only a minimal amount ofbubbles were observed on the free surface of the composition. Nointerruptions occurred on either the dispensed liquid or on the layers,which were 0.015 inch thick. After the distributor top was photoformedit was thermally treated in a convection air oven at 165° F. for 15minutes for the part to achieve its ultimate strength. It should benoted that when the dispenser was raised more than 5 mm above the freesurface of the composition, the dispensed liquid in the form of acurtain had interruptions along its width resulting in inadequatecoverage of previously photoformed layers.

1. In an apparatus for fabricating an integral three-dimensional objectby selectively exposing successive layers of a liquid photoformablecomposition to actinic radiation, said apparatus including an imagingmeans for exposing said layers, a vessel for containing a fixed amountof said composition so as to present a free surface at a substantiallyconstant position relative to said imaging means, and a movable platformdisposed within said vessel below said free surface, the improvement insaid apparatus comprising: a means to transfer a part of saidcomposition above said free surface; and a layering means for contactingthe composition transferred above said free surface and moving over saidplatform to form a layer of said composition.